Automatic volume control system



Jan. 18, 1938. H. P. SPEARIN AUTOMATIC VOLUME CONTROL SYSTEM Filed Dec. 14, 1935 2 Sheets-Sheet 1 Jan. 18, 1938. H. P. SPEAR IN 7 AUTOMATIC VOLUME CONTROL SYSTEM Filed Dec. 14, 1955 2 Sheets-Sheet 2 Int/671207" ea rzrz,

V flarazci 1 6 Patented Jan. 18, 1938 UNITED STATES PATENT OFFICE AUTOMATIC VOLUIWE CONTROL SYSTEM Harold P. Spcarin,

Chicago, 111., assignor to 5 Claims.

This invention relates to an automatic volume control system for a radio signal receiver, and more especially to such a system wherein large AVC voltages are developed without the use of separate amplification.

One object of this invention is to provide a system wherein automatic volume control voltage is obtained, without the use of a diode rectifier, in a tube circuit which provides both the automatic volume control voltage and the required tube characteristics for demodulation of a received radio frequency signal; another object of this invention is to provide an economical system which will produce relatively large voltages for automatic volume control purposes, without recourse to the use of an additional tube and circuit particularly for automatic volume control purposes; yet another object of this invention is to provide a source of automatic volume control voltage in a detector tube and associated circuit, the magnitude of which voltage is substantially equivalent to that obtained by means of an additional amplifying tube feeding into a diode or rectifier circuit; a still further object of this invention is to provide additional means in a tube prior to the detector, whereby an increased voltage for automatic volume control purposes can be obtained from the detector tube and associated circuit; other features and advantages of the invention will be apparent from the following specification and the drawings, in which- Fig. 1 is a diagrammatic View of a part of a radio signal receiving system embodying the invention; Fig. 2 is a diagrammatic view of a diode used to stabilize the system; Fig. 3 is .a diagrammatic view of a modification employing a battery to obtain greater AVC voltages; Fig. 4 is a diagrammatic view of a system wherein a rectifying tube provides a source of substantially constant voltage to replace the battery of Fig. 3; and Fig. 5 is a diagrammatic view of a portion of a radio signal receiver wherein the screen grid voltage of the detector tube is varied in accordance with the strength of the incoming signal to increase the automatic volume control voltage developed in that tube.

Modern radio signal receivers practically all incorporate an automatic volume control system wherein a DC biasing potential is developed proportional to the strength of the incoming signal and applied to the grid systems of one or more tubes having varying amplification factors. In order to have a comparatively fiat output through conditions varying from a weak signal of perhaps 10 microvolts to the strong signal of a nearby powerful broadcasting station, it is necessary to develop comparatively high voltages for automatic volume control purposes. This has generally been done by diverting a portion of the signal passing through the radio receiver, am- 5 plifying it in a tube used for this purpose alone, and then rectifying it by a diode or other similar means. Such a system develops the necessary voltage swing for good automatic volume control purposes, but is expensive in that it requires 10 either two additional tubes or a single dual purpose tube with associated circuits. The automatic volume control system disclosed herein obtains a voltage swing equivalent to the amplified automatic volume control system, yet uses neither a separate tube nor a dual purpose tube,

at least in its simplest form. The present system employs only, in addition to the apparatus necessary in the receiver, a voltage divider or potentiometer which makes use of changes in the plate current in the detector tube to develop the various DC voltages desirable for automatic volume control purposes.

In the particular embodiment of this invention illustrated in Fig. 1, a portion of a radio signal receiving system is illustrated having two radio frequency amplifying tubes l0 and II and a detector tube [2. These tubes are illustrated as pentodes, since this type of tube is more sensitive than a triode, although for the purposes of this invention a three-element tube will achieve the desired results. The three tubes shown have associated therewith tuning or tank circuits comprising inductances l3, l4 and I5 and variable condensers I6, I! and I8. These condensers are preferably ganged for simplified tuning. The radio frequency impulses picked up by the aerial l9 are developed across the primary inductance 20, which in turn develops radio frequency voltages in the tank circuit comprising the inductance l3 and the condenser IS. The voltages of the frequency to which this circuit is tuned are then developed on the grid 2! of the tube In, and the amplified signal passes from the plate 22 through the primary 23, and is transferred to the inductance M and developed on the grid 24 of the tube ll The amplified current from this tube passes from the plate 25 through the primary inductance 26, and is transferred to the inductance l5 and developed between the grid 2! and the cathode 28 of the detector tube l2. The biasing resistor 29 in the cathode circuit of this tube, in series withthe resistor 82 to the B voltage supply, keeps the grid at a negative potential and causes the tube to demodulate the radio signal carrier wave, so that an audio frequency output is had from the plate 30 of this tube. The audio frequency voltage is here shown as developed across the plate load resistor 3| and passed through the filter resistor 32 and condenser 33, whence the leads 34 and 35 apply it to the audio frequency amplifying system of the receiver, not here shown since it forms no part of the present invention.

The plate supply or B-voltage comprising battery voltage or the rectified output of a powerpack, is developed across a voltage divider here,

shown as comprising the resistors 36 and 31,,and the speaker field 38. The point 39 between the resistor 31 and the speaker field 38 is grounded, so that the voltage supply is partly above and partly below ground potential, since the oathodes 40 and 4| of the radio frequency amplifying tubes l0 and II are grounded. The B-voltage supply is thus partly positive and partly negative with respect to the cathode potential of these last-mentioned two tubes. The substantially direct current B-voltage supplied by this voltage divider system is developed across a potentiometer comprising the plate load resistor 3| and the resistors 42, 43, and 44. The point on the potentiometer between the resistors 43 and 44 is connected through the filter resistors 46 and 41, respectively, to the grids 24 and 2| of the tubes and Ill. The voltage of the point 45 on the potentiometer is thus applied at all times to the grids of the radio frequency amplifying tubes and serves to control the amplification factor of these tubes in accordance with the strength of the incoming signal, since variations in signal strength cause variations in the voltage of the point 45 with respect to ground, and consequently with respect to the cathodes 40 and 4| of the tubes In and I I. It will be noted that the B-voltage has two parallel paths, one through the potentiometer and the other through the tube l2. Both of these paths, however, have the plate load resistor 3| in common. The resistors 42, 43, and 44 are chosen of such value that under normal conditions, that is, with little or no signal being received, the point 48 is at substantially ground potential, although not connected to ground. The resistor 43 is chosen of V such size that with a normal current passing through the potentiometer the point 45 is slightly negative with respect to the point 48, and therefore with respect to ground. There is thus normally a slight negative bias voltage applied through the filter resistors 46 and 41 to the grids The current through the load resistor 3| is a resultant of the current through the resistors 42, 43, and 44, and the space current in the tube |2 flowing from the cathode 28 to the plate 30. Since the space current through the tube varies in accordance with the signal voltage impressed on the grid 21, the current through the plate load resistor 3!, and therefore necessarily the voltage developed across that resistor, also varies. When the space current in the tube increases, due to an increased signal, the increased voltage developed across the plate load resistor 3| results in a lesser voltage drop across the three resistors 42, 43, and 44. Since the proportions are fixed by the resistor values, the voltage drop across the resistor 44 will become smaller, or in other words the point 45 will become more negative with respect to ground and therefore with respect to the cathodes 40 and 4|. The voltage applied to the grids 2| and 24 of the tubes II] and II is therefore more negative than before with respect to the cathode, and the amplification factor of the tubes is decreased thus reducing the signal strenth applied to the grid 21 of the detector tube l2. In this manner variations in signal strength cause the biasing voltage applied to the grids of the radio frequency amplifying tubes to vary and thus automatic volume control is achieved.

In order to minimize the effects of undesired radio frequency voltages in the system, by-pass or filter condensers 49, 50, 5|, 52, 53, 54, and 55 are inserted in appropriate parts of the circuit. The tubes Ii], II, and I2, in addition to control grids 2|, 24, and 21, are provided with screen grids 56, 51, and 58, and with suppressor grids 59, 60, and 6|, which last-mentioned grids are here shown connected to the cathodes of the respective tubes. The voltage for the screen grid 58 of the tube I2 is supplied by the lead 62 from the point 63 in the B-voltage divider system. It is understood that the screen grids 56 and 51 are similarly supplied with proper voltages from any convenient source, not here shown. Each of the tubes In, H, and I2 is also, of course, supplied with a heater and associated circuits not here shown, in order to heat the cathodes 40, 4|, and 28 to the emitting temperature.

In the particular receiver disclosed in this embodiment of the invention, a B-voltage of,310 was used, with 100 volts drop across the speaker field 38. A screen grid voltage of 43 volts was used and the cathode bias resistor 29 developed 2.8 volts bias under normal or no-signal conditions. The plate resistor 3| had a value of megohm and the resistors 42, 43, and 44, comprising the remainder of the potentiometer system, had values of 5 megohms, A; megohm and 2 megohms, respectively. Under these conditions, a variation from zero to 1% volts on the grid 21 of the particular detector tube |2 used resulted in a voltage change or swing of 85 volts across the plate load resistor 3|. The change in voltages across the various parts of the poten tiometer system resulted in a 17 volt swing at point 45, or in other Words, when there is a 1% volt signal applied to the grid 21, the resulting change in the space current in the tube I2 causes the grids 2| and 24 in the radio frequency amplifying tubes In and H to be biased about 17 A volts more negative than before, thus cutting down the signal strength applied to the tube |2 and giving the desired automatic volume control. Where it is desired to secure a larger biasing voltage swing for the same detector grid voltage change, either a larger B-voltage may be used or a greater proportion thereof developed across the speaker field 38, that is, below ground potential. Either of these methods has its disadvantages, however, since a substantial increase in total B- voltage requires a considerable increase in power pack expense, and a different division of the B-Voltage results in less positive voltage being available for the plate of each of the tubes, with a resultant loss in amplification. Other methods for increasing the biasing voltage swing without the necessity for resorting to either of these two methods will be discussed hereafter.

In the particular embodiment of this invention illustrated in Fig. 2, the detector tube l2, in addition to the five elements shown in Fig. 1, is provided with a diode plate or rectifying element 64. This is connected by the lead 65 to the point 43 in the potentiometer system, which point is normally at substantially ground or cathode potential. In a laboratory or custom built receiver the resistors 42, 43 and 44 of the potentiometer system can be carefully proportioned so that, in connection with a particular tube and plate load resistor 3i, the point 48 is at substantially ground or cathode potential. In factory methods of production, however, it is not always possible to perfectly match these resistors with the individual tubes, and to hold them within sufiiciently close limits to keep the point 48 at the desired voltage. The introduction of the diode plate 64 serves to stabilize the voltage of this point 48 and to hold it at or very close to the cathode potential under no-signal conditions. Should the point 48 tend to be more positive than is desirable, a flow of electrons from the cathode '28 to the diode element 64 would tend to'hold this point 48 at' approximately cathode potential. The fact that a reverse fiow of electrons cannot occur enables the point 48 to swing considerably negative when a strong signal is impressed on the'tube, and thus the automatic volume control effectiveness of the system is not impaired.

In the particular embodiment of this invention illustrated in Fig. 3, the potentiometer resistor 42 is replaced by a battery 86 and smaller resistor 83. The voltage of this battery is chosen to substantially equal the voltage drop across the resistor 42 under normal or no signal conditions. When this battery 66 is used it prevents a part of the voltage swing in the plate load resistor 3| from being absorbed in the resistor 42, and instead causes practically the entire voltage swing in the resistor 3| to be developed across the resistors 43 and 44. It is clear that this method causes a much larger proportion of the plate voltage swing to be developed across the resistor 44 and applied to the grids of the radio frequency amplifying tubes. Under the same conditions as those heretofore discussed in connection with Fig. 1, the circuit of Fig. 3 causes automatic volume control voltages well in excess of 20 volts to be developed. The overall amplification of the receiver of the volume curve can thus be held more nearly flat.

In the particular embodiment of this invention illustrated in Fig. 4, another system is used to develop a larger proportion of the voltage swing in the plate load resistor 3| across the resistor 44. In this case, the resistor 42 has a separate diode tube connected in parallel therewith, and the voltage across this resistor 42 is thus held at a substantially constant value. A diode tube 61 is here shown having a cathode 68 and a plate element $9. The cathode is connected to the top of the resistor 42 through the filter or split load resistance 10, part of this resistance and the condenser 84 forming a filter system for the output of this rectifier. The plate is connected through the secondary H of a transformer and the lead 52 to the bottom of the resistor 42. The primary 13 of this transformer may either be separately fed from the alternating current source or the entire transformer may be a part of the power-pack of the receiver. This system gives increased proportions of voltage swing across the resistor 44, with increased automatic volume control action, without the necessity of a dry cell battery in the receiver.

In the particular embodiment of this invention illustrated in Fig. 5, very effective automatic volume control is achieved, since the voltage on the screen grid 58 of the detector tube I2 is varied to increase the swing of the automatic volume control voltage as a result of change in signal strength. Inthe circuit diagrammatically illustrated in this figure, the screen grid 58 is not supplied with voltage from the principal voltage divider comprising the resistor 14 and'the speaker field 38, but isinstead connected to the point E of the auxiliary voltage divider comprising the resistors I6, 11, and 18. These later named resistors are connected between B plus and ground and associated with the tube I I. The screen grid 51 of-the tube II is connected to the point 19 on this voltage divider, and the current through resistors 16 and TI and the screen grid current of the tube ll both flow through the resistor 18. -A change in the screen grid current in the tube ll thus causes a change in the voltage of the point 15. Since this is the voltage applied to the screen grid 58 of the tube l2, there is thus a change in the space current flowing through the detector tube l2, and a consequent change in the voltage drop across the plate load resistor 3 l The operation of this last-described system is substantially as follows: Under no signal conditions or light signal, the control grid 24 in the tube H is only slightly negative, and therefore a fairly large screen grid current is flowing. This screen grid current results in a relatively large voltagedrop across the resistor 18, and the point Ii-would normally be only 40- or 45 volts positive, the screen grid 58 in the detector 12 being at a like potential. A strong signal, however, causes an increase in the cathode to plate current in the detector tube [2, an increase in the voltage drop across the plate load resistor 3|, and the point 45 in the potentiometer system becomes more negative with respect to ground. Since the voltage at the point 45 is applied to the control grid 24 in the tube I I through the filter resistor 46, the grid 24 becomes more negative and cuts down the screen grid current flowing in the tube II. This results in a smaller voltage drop across the resistor 18 with a consequent shift or swing of voltages across the three resistors 16, 11, and 18, so that the point becomes more positive with respect to ground. The voltage on the screen grid 58 in the tube [2 is thus increased, causing a heavier space current to flow from the anode to the plate of the tube l 2, and thus the swing of voltage of the point 45 is increased to result in stronger automatic volume control.

While I have shown and described certain embodiments of my invention, it is to be understood that it is capable of many modifications. Changes, therefore, in the construction and arrangement may be made without departing from the spirit and scope of the invention as disclosed in the appended claims, in which it is my intention to claim all novelty inherent in my invention as broadly as permissible, in view of the prior art.

What I claim as new and desire to secure by Letters Patent, is:

1. A radio signal receiving system, including; a radio frequency amplifying tube and a detector tube, each having a cathode, grid and plate; a source of substantially direct current voltage; means connecting the plate of said detector tube to the positive end of such source, said means including a plate load resistor; gain control means for supplying a variable bias to the amplifying tube connecting said plate of said detector to the negative end of said source, said means including a portion having a substantially constant voltage drop thereacross during application of the variable bias to said amplifying tube during control action; and a connection between the grid of said amplifying tube and a point on said last-named means negative with respectto the cathode of said amplifying, tube for render? ing the gain control means efiective.

2. Apparatus of the character claimed in claim 1, wherein said detector tube also contains a rectifying element and said element is connectedto a point on said last-named means at substantially the potential of the cathode of said detector tube. v

3. Apparatus of the character claimed in claim 1, wherein said portion comprises alresistor having the opposite ends thereof maintained at a substantially constant voltage differential by means of a rectifier tube anda source f;alter-' nating current voltage. g i v 4. A radio signal receving system, including: a radio frequency amplifying tube and a detector tube, each having a cathode, grid and plate; a source of substantially direct current voltage, one terminal thereof being negative with respect to the cathode of said amplifying tube; means connecting the plate of said detector tube to the positive end of said source, said meansincluding a plate load resistor; gain control means for the amplifying tube connecting said plate of said detector to the negative end of said source, said means including a battery; and a connection betweengthe grid of said amplifying tube and a point onsaidlast named means negative with respect to the cathode of said amplifying tube for rendering the gain control means effective. 5. Aradiov signal receiving system, including: airadio frequency amplifying tube and a detectorxvtube each having a cathode, grid and plate; ,a-rsource of substantially direct current voltage, one terminal thereof being negative with respect to the cathode of said amplifying tube; means connecting the plate of said detector tube to the positive end of said source, said means including a plate load resistor; gain control means for supplying a variable bias to the amplifying tube-connecting said plate of said detector to the negative end of said source, said means in- .cluding a portion having a substantially constant voltage drop thereacross during application of the variable bias to said amplifying tube; and a connectionflbetween the grid of said amplifying tube and a point on said last named means negativewith respect to the cathode of said amplifying tube for ,rendering the gain control means effective, saidv constant voltage portion lying between saidlconnection and the plate of said detector. l

' HAROLDP. SPEARIN. 

